Structure and mechanical properties of cast alloys of the Ti-Si system
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STRUCTURE AND MECHANICAL PROPERTIES OF CAST ALLOYS OF THE Ti–Si SYSTEM M. M. Kuz’menko
UDC 669.295:669.2/8
Using the results of investigations of the structure, mechanical properties, and fracture micromechanisms of cast alloys of the Ti – Si system, we show that compositions with silicon content not higher than 2 mass % are promising for the development of heat-resistant titanium alloys having satisfactory plasticity at normal temperature.
In the traditional titanium alloys, which are capable of working at elevated temperatures, the silicon content is limited due to its solubility in α-titanium: 0.31 – 0.54 mass % within the temperature range 750 – 860°C. In β-titanium, 3 mass % of silicon can be dissolved at an eutectic temperature of 1330°C and even less at lower temperatures. At 860°C, the eutectoid transformation β → α + Ti5 Si3 takes place in alloys of the Ti – Si system [1]. Titanium composites based on the Ti – Si system with silicon content exceeding its thermodynamically stable concentration in the solid solution are considered promising. In such composites, the Ti5 Si3 refractory phase is formed, which naturally hardens the titanium matrix due to crystallization in the course of cooling. The results of investigations of foreign [2, 3] and Ukrainian [4 – 6] scientists show that the alloys of this system possess a heightened refractoriness and high-temperature strength. However, the laws of structural and phase transformations in these alloys, the micromechanisms of their fracture, and mechanical properties remain unclarified, despite previous studies [7 – 10]. In the present work, we investigate the properties of binary alloys of the Ti – Si system in the cast state depending on the silicon content and testing temperature in order to develop, on this basis, more heat-resistant compositions. Materials and Experimental Procedure The tested alloys (see Table 1) were melted by the plasma-arc method in argon with preliminary vacuumization of the melting chamber. We used VT1-0 commercial titanium and KR1 silicon. The melt temperature prior to pouring out to the mould was from 1620 to 1660°C. We poured out the liquid metal to a graphite crucible in the melting chamber. The alloy was cooled to 600°C in the chamber and further outside it. We obtained castings of diameter 60 mm, height 120 – 160 mm, and mass 2 – 3 kg, containing many admixtures, which affect substantially the characteristics of strength and plasticity. The microstructure and fracture mechanisms of these alloys were studied with the help of a Superprobe-733 scanning electron microscope. The mechanical properties were measured under point bending of prismatic specimens (of size 2.5 × 5 × 50 mm) at 20°C and tension of cylindrical specimens (of length 38 mm and working diameter 3 mm) at 20 – 600°C. Frantsevych Institute for Problems of Materials Science, Ukrainian Academy of Sciences, Kyiv, Ukraine. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 1, pp. 45 – 48, January – February, 2008. Original article submitted September 5, 20
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